A great deal of data on the heats of formation of
various hydrates has been compiled i n the J.A.N.A.F. and
other tables such as the National Bureau of Standards
circulars. Comparison of the heat of f ormation of a hydrate
with that of the corresponding anhydrate exposes anomalies
i n a surprising number of cases. Some of the results are
so discordant that i t is apparent that one or the other
value is seriously mistaken.
No attempt has been made i n this work to determine
which value may be correct, but measurements have been made
of the difference between these two values. The procedure
adopted has been to dissolve the hydrate and the anhydrate,
to achieve the same final concentration of the compound in
solution, and so to measure the difference in heats of
solution ..
Measurements were made at OOC in a modified Bunsen
ice calorimeter, well insulated and surrounded by an icewater
mixture .
The observed differences in heats of solut ion were
corrected t o 25°0 by using appropriate heat capacity data.
These differences offer a direct measure of the enthalpy
involved in binding a mole of water into the crystal structure and so should shed light on the nature of
binding involved.
The following hydrates were studied :
MgS04.nH20 (n = 1,4,7), MnC12.nH20 (n = 1, 2),
LiI. nH20 (n = 1,3), MnS04. nH20 (n = 1,4),
CaC12. nH20 (n = 2,6) , K2C03.1~H20, LiCl.H20, LiBr.2H20,
CdC12.2t H2o, and N2H4eH20.